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Sega Neptune

During 1994 and 1995, SEGA would have two separate teams working on two different consoles: the Saturn and the Neptune. They had said to the teams very early on that only one console would actually make it through and the other would be scrapped, as if encouraging competition and therefore innovation. Until near the end of this period, everyone thought that the Saturn would be the one to come out and that the Neptune would be a complete flop. However, the Neptune team had been keeping most of their ideas under the radar, and once their final prototype was demonstrated to the executives the situation reversed completely. It turned out that the Neptune, retaining its legacy of BLAST PROCESSING from the Genesis, was able to do fairly good 3D in software. It integrated the technology of not only the Genesis and 32X but also the Sega CD, but throwing out the 32X's 4MB ROM limit and the SCD's ridiculous loading times. Using a CD drive for its media, it was able to play all Sega CD games by itself, and Genesis and 32X games were also fully playable with a cheap $20 add-on, which would come as a free extra for the first 10,000 units sold. Even with all this, the cost per unit was still far cheaper than that of the Saturn. So the Saturn team were told that their hardware 3D technology wasn't yet ready and should be saved for later - this would become the Dreamcast. The Neptune was finally released in early 1996.

Technical specifications

One contiguous RAM chip, 4MB in size. This would be divided by a separate memory mapping chip when compatibility mode was enabled (for Genesis, 32X and SCD games).

32kB of battery-backed RAM, four times the size of the Sega CD's equivalent, used for storing game data. The Neptune would be 100% compatible with the same Backup RAM Carts that the Sega CD used.

32-bit address space, mapping the Neptune's new RAM space and system functions to addresses unused by the 32X.

Main processors:

Two SH2 processors running at 26MHz. These would be used by 32X games. Neptune games would usually use these to control audio/video loading, decompression and rendering.

Two 68k processors running at 13MHz (exactly half that of the SH2 as it runs from a hardware divider from the SH2 clock for convenient synchronisation). These would be used by Sega CD games. For Genesis and 32X games, one would be used and the other would be idle. Neptune games would usually use these to control game logic and the original Genesis audio/video processors.

One Z80 running at 3.58 MHz (the same speed as in the original Genesis to ensure sound quality is retained). Some games, particularly Genesis games, would use this to control the YM2612 audio processor.

Sound chips:

One YM2612 chip and one SN76489 chip as used on the Genesis, which can each be controlled either from the Z80 or the primary 68k.

The SN76489 chip has four channels, however the original Genesis can only ever access three of them at once due to bad design. The Neptune, however, can access all four at once. The total number of channels accessible to the Neptune is therefore 22: 6 YM2612 + 4 SN76489 + 8 RF5C164 + 4 PWM

One YM7101, plus whatever the Sega CD custom video chip was, for displaying Genesis, SCD and 32X graphics. These chips, like everything else, are also accessible to Neptune games for masochistic programmers, but it's far easier to just use the 32X's framebuffer-based graphics exclusively and be done with it.

Compare that to the 1×-speed, yes, just 1×-speed CD-ROM of the original Sega CD.

Sega CD games couldn't play CDDA (pre-recorded) music and load data at the same time, which is evident in Sonic CD. However, because of the faster speed and the abundance of processors and RAM, one can quite easily compress their music in a format such as MP3 and run a realtime decoder on one of the SH2 processors. This decoder can then load its data in segments at a low priority and output directly via the DAC. This enables prerecorded music to be playing (and even looping) while data is being loaded.

Launch titles

Sonic 4

Sonic the Hedgehog 4 would be released as a launch title for the Neptune. It would be a mostly 2D version of what we know as Sonic Adventure, except without Big the Cat and with optional adventure fields. Playable characters would be Sonic, Tails, Knuckles and Tikal. Each character would be able to play through the same kind of gameplay as Sonic, just like in Sonic 3, but would have the Adventure elements added in as optional extras. Some parts of the game would involve 3D elements, such as the part where you ride on Tails' plane. The game would be able to transfer Chao between the Chao Garden and the VMUs available from the Saturn controller.

Sonic RPG

Sonic RPG was released in 1997 for the Neptune. It was an action-RPG hybrid that combined fast paced battles with elements of traditional Sonic games and RPGs. The plot of the game revolved around Dr. Eggman seizing an ancient relic called the Crystal Orb to rival the power of the Chaos Emeralds. The game was known for it's extensive use of the 3D software rending capabilities, as well as maximizing the sound chips.

Sonic X-Treme

Due to the great success of the Neptune and Sonic 4, the most importantSonic X-Treme developers were never taken off the project, and the game was also given a much longer timescale, allowing it to come to completion. It would eventually be released as a launch title for the Dreamcast in 1998. It would be the second game, after Sonic 3D Blast, to feature voxel-based 3D worlds.

Sonic 5

Sonic the Hedgehog 5 would be released for the Dreamcast in 1999 as a version of what we know as Sonic Adventure 2. Its Knuckles/Rouge and Tails/Eggman stages would be roughly unchanged from the version we know, though the Sonic/Shadow action stages would have been made in 2.5D. That is, instead of being fully 3D where you'd hold forward almost the entire time, they would be arranged as a 2D grid of 3D chunks which you would navigate left to right like the game's predecessors. This engine allowed the Sonic/Shadow levels to be much longer and more complex, on par with Sonic 3 & Knuckles and Sonic 4.